1. Field of the Invention
This invention relates to ion accelerators (making use of principles of tandem acceleration methods) to be use in manufacturing semiconductors.
2. Description of the Prior Art
Accompanying the high accumulation of semiconductors in recent years, increasing importance is given to high energy implant process which can freely control impurity profile in the interior of silicon substrates. Thus, present tandem acceleration principles are used most widely as a method of accelerating ions to high energy and implanting them in silicon substrates. Tandem acceleration principles are well known and are described in U.S. Pat. No. 3,353,107 and elsewhere. In this tandem acceleration principle, a negative ion beam is produced by combining a positive ion source and a charge exchange cell, or by using a sputter type negative ion source. This negative ion beam is directed into an accelerator terminal which is maintained at high positive voltage, injection-accelerated, and accelerated to the terminal voltage. Then, electrons are stripped from this accelerated negative ion beam in the accelerator terminal by causing it to pass through a gas or thin foil, and the beam is converted to a positive ion beam. This positive ion beam is accelerated again to ground potential from the accelerator terminal maintained at high positive potential and acquires its final energy.
At this time the final energy E.sub.tot (eV) of the ions may be shown as EQU E.sub.tot (eV)=E.sub.inj .times.Q X (N+1)V.sub.ter
where E.sub.inj (eV) is the injection energy into the accelerator, V.sub.ter (Volt) is the terminal potential, N is the charge number of the positive ions and Q (Coulomb) is the magnitude of the electronic charge, and one can use the impressed terminal voltage efficiently in accelerating the particles.
As an example of an actual apparatus which uses this tandem principle, the construction of a Genus Inc. model G1500 high energy ion implanting apparatus, modified by omitting a pre-acceleration tube now used on the model G1500, is shown in FIG. 1. For an understanding of such prior art devices reference is also made to U.S. Pat. No. 4,980,556.
In this apparatus positive ions are produced by a hot-cathode PIG ion source 1. These positive ions are extracted as a beam by impressing a high positive voltage on the ion source. The extracted positive ion beam collides with magnesium vapor when passing through a charge exchange cell 2 which is set up immediately after the extraction electrode system, and some of the positive ions in the positive ion beam pick up two electrons from the magnesium and are converted to a negative ion beam.
After passing through the charge exchange cell 2, this beam is analyzed according to the charge state and the mass of the ions therein by means of a 90-degree analyzing magnet 3, and only the desired negative ions are injected into the tandem accelerator 5.
This mass-analyzed negative ion beam, by means of the pre-Q lens 4 which is furnished at the entrance aperture part of the low-energy acceleration tube 6 of the tandem accelerator 5, receives a focusing action such as to create a beam waist at the center of the stripper canal 7 which is provided in the tandem accelerator terminal part. At this time, the negative ion beam is simultaneously accelerated towards the tandem accelerator terminal part which is maintained at a high positive potential.
When this accelerated negative ion beam passes through the stripper canal 7, it loses orbital electrons by colliding with nitrogen gas which is introduced into the stripper canal 7, and is converted again into a positive ion beam. At this time, the distribution of charge states is determined by the energy of the collisions, and more multi-charged ions are produced at the higher collision energy. An example of this charge state distribution is shown in FIG. 2 for the case of boron.
The positive ion beam which is thus obtained is directed towards ground potential from the tandem accelerator terminal, and is again accelerated in passing through the high-energy acceleration tube 8.
The beam which thus has its final energy receives a further focusing action by means of the post-Q-lens 9, the desired charge state is selected by means of the post-analyzing magnet 10, and is introduced into a process chamber which is provided with a target.
However, in this tandem accelerating method the useful beam current which reaches the target is regulated by the charge state distribution which arises in the accelerator terminal, and therefore, as shown in FIG. 3 for the case of boron as an example, for final energy in the range of 500 keV and below the defect occurs that beam current is drastically reduced. Moreover, the negative ion yield is generally lower by 5-15%, and therefore the defect occurs that efficiency of use of the beam is reduced. It is the object of this invention to solve these defects.